Process for using chromic acid in the preparation of dispersion strengthened refractory metal alloys



United States Patent 3,473,914 PROCESS FOR USING CHROMIC ACID IN THE PREPARATION OF DISPERSION STRENGTH- ENED REFRACTORY METAL ALLOYS Christian E. Rick, Wilmington, Del., assignor, by mesne assignments, to Fansteel Inc., a corporation of New York No Drawing. Filed June 12, 1967, Ser. No. 645,483 Int. Cl. B22f 9/00; C22c 39/00, 33/02 U.S. Cl. 75.5 Claims ABSTRACT OF THE DISCLOSURE This invention comprises a process for preparing dispersion strengthened refractory metal alloys having a metal oxide phase and a continuous metallic phase. In particular it comprises an improved process whereby a solution containing chromic acid is used to dissolve scrap metal composed of metals desired in the continuous metallic phase. The metals in solution are separated from the solution and then reduced to the metallic state, with at least part of the chromium in the final metallic phase being the chromium added to the solution as chromic acid.

DESCRIPTION OF THE INVENTION The present invention consists of a process for preparing a metallurgical powder to be compacted and worked to produce dispersion strengthened refractory metal alloys having a metal oxide phase and a continuous metallic phase wherein the components of the continuous metallic phase are precipitated as oxidic compounds before being reduced to the metallic state. The invention comprises dissolving at least a portion of the alloying metals in a solution containing chromic acid to form a first solution, separating the oxidic metal compounds from said first solution, and thereafter reducing said metal compounds to the metallic state. The metals dissolved in the first solution may be precipitated by adding a second neutralizing solution comprising ammonium carbonate. The solution containing chromic acid may also contain nitric acid to aid in dissolving the metals.

More particularly, the invention comprises, in a process for the preparation of dispersion strengthened refractory alloys having (1) a finely divided, dispersed, refractory, metal oxide phase and (2) a continuous metallic phase consisting of chromium alloyed with at least one metal from the group consisting of iron, cobalt and nickel, said metallic phase containing not more than 10% of other alloying ingredients, wherein the components of said metallic phase are precipitated from aqueous solution as oxide phase and subsequently reduced to the metallic state, the improvement comprising supplying at least a portion of the chromium component by adding chromic acid to aqueous nitric acid to form an aqueous chromic acid-nitric acid mixture, and then dissolving at least enough metal selected from said metallic components of said metallic phase, in the aqueous nitric acidchromic acid mixture to reduce the chromium in said chromic acid to the tri-valent state prior to the precipitation of said oxidic compounds .of the metallic phase elements.

In a preferred embodiment the metal being dissolved is dispersion modified metal scrap of composition similar to the product being made or compatible therewith.

The inveniton is concerned with the composite materials known as dispersion strengthened and dispersion modified refractory alloys. These alloys are particularly suited for applications where the maintenance of a high degree of mechanical strength at high temperature is required. The continuous metallic phase has as major components iron cobalt or nickel and chromium with the preferred chromium content ranging from 10 to 30% by weight of the metal matrix. Minor components may include molybdenum, tungsten, manganese, titanium, zirconium, vanadium, columbiam, other metals having boiling points above 1200 C. and carbonplus the usual traces. The metal oxide dispersed phase comprises very finely divided particles of 'high melting stable oxides such as Y O CaO, La O BeO, ThO MgO, V0 HfO' C602, A1203, ZIOZ, BaO, TIOZ, Slog, T3205, Nboz, Cr O and mixed oxides, particular the spinels. Thoria, magnesia, zirconia and calcium oxide are especially applicable. These may be dispersed in the solution of alloy elements as oxides or other oxidic compounds or be dissolved and then co-precipitated from the solution along with oxidic compounds of the alloying elements. In either case the result is an intimate mixture of very fine particles of the oxidic compounds of both classes. Precipitation may be accomplished by neutralization of the acid solution or by thermal decomposition of the whole solution as, for example, by spray drying and calcination, or by a combination of such procedures. The actual refractory oxide particles may be thermally derived from the oxidic compounds at a later stage. Some of the oxides such as CaO and BaO are water soluble and they may be precipitated as oxalates, carbonates, etc., to be later decomposed into the oxide. The ultimate oxides are well dispersed among the oxides of the alloying metals prior to reduction of the latter.

The mixture of oxides thus obtained is then reduced at elevated temperatures by reaction with such agents as hydrogen, methane or other hydrocarbons, and carbon in known manner. The resulting metal powder mixed with the fine stable oxide, which is not reduced by the conditions chosen, is compacted and worked by known powder metallurgy operations to produce a dense composite product wherein the fine oxide particles are dispersed in a continuous metallic phase that has greater resistance to deformation at high temperature than corresponding products without the dispersed phase.

Several problems are associated with the preparation of these materials, among which is the introduction of the chromium. The relatively expensive Cr O is the preferred oxide to have present in the oxide mixture prior to reduction to the metallic state. Cr O however, is not readily dissolved. On the other hand CrO or chromic acid, is available in pure form and is easily dissolved in water or acid solution. CrOg, thus qualifies as a convenient source of chromium, but its use poses still other problems. Hexavalent chromium is not desired in the precipitated oxide mixture because of certain deleterious effects arising on heating the mixture nor is it co-precipitated on neutralization of the acid. The process of this invention fortunately solves the problem of reducing the chromium valence and at the same time enhances the rate of solution of metallic raw materials. The enhanced rate of solution of metallic raw materials is especially advantageous in a process where scrap metal containing iron, cobalt or nickel is used.

Although the alloying metals will dissolve in nitric acid, the presence of chromic acid speeds solution and reduces loss of nitric acid as solvent proper since chromic acid is consumed as an oxidizing agent. Furthermore, after serving as an oxidizing agent, the chromium remains in solution in the most desired tri-valent form to become, without loss, part of the finished product. It is necessary, however, to regulate the amount of chromic acid used relative to the amount of metals to be dissolved so that all chromic acid is reduced to the tri-valent state while in solution. The amount of chromic acid necessary should be checked experimentally since, at elevated temperatures, the nitric acid will be reduced to some extent by the metal and more than the theoretical quantity of metal will be required. Of course, when all the iron group metals required are supplied in the metallic state, they will be in excess, since the final product is to be not more than 30% Cr.

The chromic acid-nitric acid mixture alone may be used to dissolve the metal or the solution may also contain various amounts of the component metal salts. Iron, cobalt and nickel nitrates and other soluble compounds which will go into nitrate solution may be added before or after the chromic acid reaction is undertaken. Such compounds in addition to the nitrate are the carbonates, oxalates, hydrous oxides, etc. The metals forming the refractory oxide dispersed phase may be dissolved in the acid solution at any time if co-precipitation is desired, or the insoluble oxidic compounds may be dispersed in the clear or clarified solution. In certain cases after dissolving the metal as when pure nickel thoria is used, the filtration step may be dispensed with so that the thoria also is recovered, becoming part of the product. In any case, when the solution is ready for clarification and precipitation, it should be checked for the presence of hexavalent chromium and any that remains should be reduced. If necessary, the final solution is adjusted to a composition corresponding, in terms of reducible metals and refractory oxide present, to the final composite alloy. The final product should contain from .01 to 10% by weight refractory oxide, preferably .1 to 5%. The desired particle size is well recognized in the art and preferably ranges from 5 to 150 millimicrons in average diameter.

The unique combined utilization of scrap metal with the chromic acid source of chromium of this invention offers economy and convenience and also solves the problem of reducing the chromic acid to the desired tri-valent state in solution.

The following examples illustrate specific applications of the invention.

Example I This example illustrates the use of chromic acid in the preparation of a composite alloy having the following approximate composition:

78% Ni, 19% Cr, 2.5% ThO residual Mo, and trace elements.

In the process for preparing the above alloy, 21 metal ion-containing solution of 800 lbs. Ni(NO -6H O, 300 lbs. Cr(NO -9H O, and 420 lbs. of water is first obtained as follows:

1) A first solution is prepared by digesting 225 lbs. of one inch pieces of previously degreased and pickled scrap sheet metal analyzing:

89.2% Ni, 4.8% Cr, 2.9% M0, 3.0% thoria and oxygen, with aqueous acid consisting of 600 lbs. HNO and 73.5 lbs. chromic acid (calculated as CrO (2) After clarification filtration of the resulting solution in 1) above, 10 lbs. of Th(NO -4H O is added to the solution. The density of the first solution at 25 C. is 1.48 gms./cc.

(3) A second solution of ammonium carbonate is prepared by dissolving:

600 lbs. of ammonium bicarbonate in 1040 lbs. water and then adding 488 lbs. of aqueous ammonium hydroxide containing 28% free ammonia.

(4) The first and second solutions prepared above are mixed together to form a co-precipitate with the rates of addition of each solution being controlled to maintain the pH of the mixed solution in the tank at about 7.0.

In the particular embodiment of the invention illustrated in this example the first and second solutions in step 4 above are fed into a conical bottom tank having a bottom outlet connected to the inlet of a centrifugal pump. The outlet of the pump is attached to a line having two inlet lines attached through T joints. The outlet line .4 returns to the tank through one valve for recycling the charge and through another valve leads to a filter press.

The tank contains a heel of filtrate from a previous batch which is recycled by means of the pump. Regulated amounts of the first and second solutions as prepared above are added to the recycle stream with the rates of addition being controlled by flow meters to maintain the pH of the solution in the tank at about 7.0. The solutions are fed into the tank over a period of about 3% hours, at the end of which time all of the first salt solution and 1135 lbs. of the ammonium carbonate solution are utilized.

After the first and second solutions have been mixed in the tank, the resulting product slurry is circulated by the pump for a few minutes and then pumped to the filter. The precipitate is filtered, washed with water, dried, calcined for 4 hours at 400 C., and ground in a hammer mill.

Seventy-five lbs. of the calcined metal oxide powder prepared above is mixed with 9 lbs. of carbon black (Raven 15, a product of Columbium Carbon Company) in a twin shell blender for two hours. The mixture is then placed on inch deep trays and inserted into a reduction chamber. The chamber is heated to 150 C. for 11 hours and then to 400 C. for 4 hours in a circulating atmosphere of purified hydrogen containing 2 volume percent of methane. At this point the NiO is reduced.

The temperature of the reducing chamber is then raised to 940 C. and held for 48 hours in a pure hydrogen atmosphere to reduce the chromium and eliminate excess carbon.

The resulting alloy powder consists of about:

78.5 Ni, 19.0% Cr, and 2.4% ThO and is useful in preparing dispersion-modified high temperature alloy metal articles.

Example 2 This example illustrates the use of chromic acid in the preparation of an iron-chromium alloy containing finely dispersed ThO In the process of preparing the above alloy, degreased and pickled iron and iron-chromium scrap containing:

150 lbs. iron and 20 lbs. chromium is digested with an aqueous solution of 500 lbs. HNO and 70 lbs. chromic acid.

After clarification filtration, 8.5 lbs. Th(NO '4H O is added to the solution to give a first solution containing:

730 lbs. Fe(NO -6H O, 422 lbs. Cr (NO -9H O, and 8.5 lbs. Th(NO -2H O in about 300 gallons of solution.

A second solution of about 300 gallons is made in water of ammonium carbonate [(NH CO to a concentration of 4.0 normal.

The first and second solutions prepared above are then fed into a precipitation T mixing system similar to that of Example 1. The rate of addition of each solution is controlled by flowmeters so that a mixed solution having a. pH of 7.0 is obtained throughout an addition period of about 4 hours. The slurry is filtered, and the precipitate washed, dried, calcined at 450 C. for 4 hours and then ground in a hammer mill.

The resulting metal oxide powder is then split into quarters and to each quarter is added 9 lbs. of carbon black (Raven 15 as in Example 1). The metal oxide, carbon mixture is then mixed in a twin shell blender and charged into trays in a reduction chamber. The chamber is then heated to 400 C. and held for 4 hours in a circulating atmosphere of purified hydrogen containing 2 volume percent methane. At this point the iron oxide is reduced.

The temperature is then raised to 950 C. and held for 48 hours in an atmosphere of pure hydrogen to reduce the chromium and eliminate excess carbon.

The resulting alloy powder consists of about:

70.6% Fe, 27.4% Cr, and 2% ThO and is useful for fabrication by powder metallurgical procedures into dense metal articles having improved strength and oxidation resistance.

Since many different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific illustrations except to the extent defined in the following claims.

I claim: 1. In a process for preparing a metallurgical powder to be compacted and worked to produce dispersion strengthened refractory metal alloys having a metal oxide phase and a continuous metallic phase consisting essentially of chromium and at least one component selected from the group consisting of nickel, iron, and cobalt wherein the components of the continuous metallic phase are precipitated as oxidic compounds before being reduced to the metallic state, the improvement comprising:

dissolving at least a portion of the alloying metals in a solution consisting essentially of a mixture of chromic acid and nitric acid, to form a first solution;

precipitating the oxidic metal compounds from said first solution;

and thereafter reducing said metal compounds to the metallic state.

2. A process as in claim 1 wherein the metals dissolved in said first solution are precipitated by adding a second neutralizing solution.

3. A process as in claim 2 wherein said second neutralizing solution is aqueous ammonium carbonate.

4. In a process for preparing a metallurgical powder to be compacted and worked to produce dispersion strengthened refractory alloys having a finely divided, dispersed, refractory metal oxide phase and a continuous metallic phase consisting of chromium alloyed with at least one metal selected from the group consisting of iron, cobalt, and nickel, said metallic phase containing not more than 10% of other alloying ingredients, wherein the components of said metallic phase are precipitated from an aqueous solution thereof as oxidic compounds and subsequently reduced to the metallic state, the improvement comprising supplying at least a portion of the chromium component of said metallic phase by adding chromic acid to aqueous nitric acid to form an aqueous chromic acid-nitric acid mixture and then dis solving at least enough metal selected from said metallic components of said metallic phase in said acid mixture to reduce the chromium in said chromic acid to the trivalent state prior to the precipitation of said oxidic compounds of the metallic phase elements.

5. The process of claim 4 wherein the metal being dissolved in said aqueous chromic acid-nitric acid mixture is dispersion modified metal scrap having substantially the composition of the product being made.

References Cited UNITED STATES PATENTS 3,386,814 6/1968 Alexander et al. 0. 5 3,388,010 6/1968 Stuart et a] 75-0. 5

L. DEWAYNE RUTLEDGE, Primary Examiner T. R. FRYE, Assistant Examiner US. Cl. X.R. 

